A lithographic apparatus is a machine that may be used to apply a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning structure, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “patterning structure” used herein should be broadly interpreted as referring to structure that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.
Patterning structure may be transmissive or reflective. Examples of patterning structure include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned. In each example of patterning structure, the support structure may be a frame or table, for example, which may be fixed or movable as required and which may ensure that the patterning structure is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning structure.”
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system”.
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”.
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
It may be desired to correctly align the substrate before exposing it, e.g. to ensure accurate projection of the functional features. Conventionally this is achieved using the apparatus shown in FIG. 2. Complimentary alignment marks M1, M2, and substrate marks P1, P2 are present on a mask and substrate respectively and an alignment system is used to detect alignment. Examples of alignment systems are a conventional through the lens alignment system and also the alignment method and apparatus described in co-pending European Patent Application Nos. 02251440 and 02250235. A mark is commonly on the front side of the substrate, but could also be on the back side of the substrate. Marks on the back side of the substrate are used, for example, when exposure is to take place on both sides of the substrate. This occurs particularly in the manufacture of micro electromechanical systems (MEMS) or micro opto-electromechanical systems (MOEMS). When the substrate marks P1 and P2 are on the back surface of the substrate they are re-imaged by front to back side alignment optics 22 at the side of the substrate W to form an image Pi as shown for P2 in FIG. 2 (P1 would be re-imaged by another branch of the front to back side alignment optics) of the accompanying drawings. The front to back side alignment optics, together with the alignment system AS are used to determine the relative position of marks on the front side of the substrate to marks on the back side of the substrate. This enables functional features exposed on the front side of the substrate to be correctly lined up with functional features exposed on the back side of the substrate.
Using conventional front to back side alignment optics a mirror image of the substrate mark is projected into the image window of the front to back side alignment optics. This mirror image is the image used for alignment described above and therefore its position relative to the actual position of the substrate mark must be accurately known. In particular, the optical axis of the front to the back side alignment optics around which the mirror image is flipped must be accurately known. Any error in the optical axis will result in twice the error in the measurement of the substrate position.
Furthermore, due to the mirroring in the optics the rotation of the image of a mark on the back side of the substrate is opposite to the rotation of the front side of the substrate. This can lead to problems during fine alignment if not taken into account.